Fifteen years old it's very late to introduce cryptocurrencies to a child. Why are you waiting a such old age? Your child is not clever enough to understand the basis of cryptography?  If your child knows what money is he should know what cryptomoney is too.

That’s not true, 15 years of age is not late for anyone to learn  Everything they want to know about cryptocurrency. Moreover, a lot of people who are here are already in their 30s and 40s when they got to know about cryptocurrency, but they still learnt about it and they are good at it now, with some of them becoming programmers and developers in blockchain, and also others becoming good investors and traders, and making lots of money to even be able to take care of themselves and their family.

So, the age doesn’t even matter, a child can learn about cryptocurrency at any age. I believe that as long as it is a brilliant child, they would always be able to come up with new ideas or innovations that will profit them in the future. Just like the two kids that were able to start up Ethereum mining and it became a success and they’re making a lot of money from it.

Welcome to the forum, it's good to know that you are worried about your children's future. I would suggest you first acquire the complete information then invest in savings. Must be known that the regulatory rules aren't against it.
Also, the future of these sites is always uncertain so we can't rely on them completely. But what I can suggest here is to go for hardware wallets. As they are more reliable.